You can stay in touch a bit. You can treat her like a friend - stay in touch on Facebook, chat sometimes. Just not a super close friend, because that is too awkward with regard to your brother.

But yeah - she's not going to be part of your family anymore, and there's no way around that. It's a really sad part of divorce, and you're right to grieve it.

Lots of divorces in my family. I've stayed friends with all of the ex's. They don't come for holidays anymore, but if it's a kids event everyone is there or if I go into town to visit my family, I always at least have breakfast with them.

yes, try to stay neutral and don't burn any bridges or say anything you'll regret should they reunite.

It looks like my brother will be divorcing--his choice. She wants to keep trying. But they have been miserable. I feel sad that this person I have known for years and years will suddenly be gone on Christmas. It's so sad. Do you have friendships with former in laws? Is there anything I can do for her that might help?